This invention relates to ultrasonic atomizers for the atomization of waste sulfuric acid in cracking furnaces, as well as to a furnace equipped with these ultransonic atomizers.
In a number of chemical processes, waste sulfuric acids containing varying amounts of impurities are obtained, which latter can be removed only by expensive procedures. Therefore, these acids, containing besides water primarily organic compounds, e.g. sulfonic acids, sulfates, such as, for example ammonium sulfate or metallic sulfates, or metallic oxides, are split reductively at temperatures of between about 850.degree. and 1250.degree. C., forming an SO.sub.2 -containing cracked gas which can be conventionally processed according to the contact method to obtain concentrated sulfuric acid or oleum. The thermal energy required for the cracking step is generated by the combustion of oil or heating gas in the cracking furnace.
Heretofore, cracking yields of up to 98% have been attained, i.e. 98% of the hexavalent sulfur contained in the waste sulfuric acid is present, after the cracking step, at sulfur dioxide, and the remainder is further in the hexavalent form as sulfuric acid vapor and/or sulfur trioxide. Besides, the cracked gases are largely laden with ashes produced by the combustion of the contaminants contained in the waste sulfuric acids. Thus, the hot cracking gases must be cooled and cleaned before they are fed to the sulfuric acid plant. In the cooling of the cracked gases from, for example, about 1000.degree. C. to about 350.degree. C., the waste heat thereof is generally utilized for air heating and/or high-pressure steam generation in air preheaters or waste heat boilers. It was found that the two percent of unreduced sulfuric acid and in some cases sulfuric acid compounds in the cracked gases, together with the other gas impurities, can lead to corrosive attacks on the air preheaters or waste heat boilers. During the subsequent gas scrubbing step, the hexavalent sulfur compounds contained in the cracked gas enter at least partially into the scrubbing water and thus are lost for the sulfuric acid recovery. Consequently, there is the need for increasing the cracking yield to an optimum value lying almost at 100%.
In order to introduce the waste sulfuric acid into the cracking furnace, air atomizer nozzles of a large cross section have heretofore been preferably employed for the feeding and discharging of the acid, since most of the waste sulfuric acids contain considerable impurities in the form of solid substances and/or polymerization products. Because of these contaminants, pure pressure atomizer nozzles tend to clog, resulting in disturbances in the operation. With the heretofore utilized air atomizer nozzles, it has been necessary to use, for throughput efficiencies of up to 20 tons of acid per hour and more, 0.5 to 0.8 Mn.sup.3 of air under a pressure of 5000 mm. H.sub.2 O column per kg. of acid, depending on the constitution of the acid, in order to atomize the acid. In this process, droplet sizes of between 400 and 600 .mu. could be obtained, with the most frequent droplet size being 500 .mu.. The disadvantage which is particularly significant for the further processing of the cracked gas to sulfuric acid is, in the air atomization method, the dilution of the cracked gas, resulting in increased expenditures for extra apparatus in the sulfuric acid portion of the total plant. In contrast thereto, if the excess of air is reduced at the oil burners in order to compensate for this increased consumption of air, a larger furnace space is required for complete combustion, and the cracking process, with irregular acid atomization, becomes more susceptible to breakdown. An improvement of the yield of the cracking process by increasing the residence time in the furnace leads perforce to an increase in furnace volume and thus to rising initial investment costs.
Starting with the above-explained problem, the present invention resides in the provision of an atomizer and a cracking furnace for waste sulfuric acid, wherein, with a reduced amount of atomizing air, or without the use of atomizing air, a cracking yield of almost 100% is attained. In particular, this objective is to be produced by optimizing the atomization of the acid and the fuel combustion in the furnace.